The invention relates to stock washers for removing impurities or other substances from a porous mat disposed over a vacuum head.
Various industrial processes often require that a mass of porous material be washed in order to remove chemicals or other impurities. For example, this need appears in the sugar industry, where sugar is washed from bagasse; in the textile industry, where excess dyes are washed from the fabric; in mining, where impurities are washed from ore; and in the paper industry, as follows.
In a standard paper production line, wood chips are cooked with chemicals in aqueous solution, the precise composition of the cooking chemicals depending on the particular process. This step, normally carried out in a digester under heat and pressure, breaks down the wood by dissolving the organic compounds that hold the cellulose fibers together.
The mixture of pulp, spent cooking chemicals, and organic materials, collectively known as stock, is then fed to a series of washers. The most common type of washer system includes a rotary vacuum drum onto which the stock is spread. The drum is perforated, and a vacuum maintained inside causes the separation of liquid from the pulp. The mixture assumes the form of a pulp mat which is still impregnated with chemicals and organics. A washer usually disposed above, and extending axially along the drum, directs water at and through the pulp mat to remove these substances. A typical installation would use three washer drums in sequence, with wash water being flowed counter-current to the direction of the pulp movement so that the final washing stage uses clean water. An additional washing stage to remove bleaching chemicals is required if the washed pulp is subsequently bleached. A hood is placed over the drum and washers to prevent steam energy from being lost and to protect workers from the chemicals.
The effluent from the washers, comprising water, spent cooking chemicals, and organic materials, is referred to as liquor. In a kraft (or sulfate) process, it is called black liquor; in a sulfite process, red liquor. Typical liquor contains approximately 15% solid material. It is desirable to separate this solid material from the water to allow reuse of the inorganic pulping chemicals, and to eliminate the environmental problem of disposing of the liquor.
Evaporation is the standard separation method, with the liquor being passed through a series of evaporators, in which steam is passed counter-current to the liquor flow. In this way, the liquor is concentrated until it contains approximately 60% solids, at which point it is burnt in a boiler. The organic materials provide the fuel to generate the steam, and the inorganic chemicals smelt out the bottom of the boiler. In a typical paper mill, the steam from the liquor recovery part of the cycle supplies most of the mill's steam needs.
It is apparent that the more dilute the liquor, the more energy must be expended in evaporating the water in order to recover the solids. This is energy that is therefore unavailable for other energy needs of the paper mill. At the same time, it is necessary to efficiently remove the chemicals from the pulp to provide a satisfactorily clean pulp. A thorough washing militates toward the formation of dilute liquors.
One existing washer is a washer sold under the name Uniflow by Comarco Industrial, Inc. of Beaverton, Oreg. This washer is disclosed in U.S. Pat. No. 4,205,541 and includes an elongate chamber placed axially above the drum. The chamber has an upper region comprising a flat plate with a gutter down its middle and a number of holes on the vertical sides of the gutter for allowing water which covers the plate to spill into the lower portion of the chamber. The lower portion of the chamber is narrow and structured so that a head of water can build up to give a static pressure for the water which will leave the chamber through a slot along the bottom of the chamber. The pressure of the water built up in the chamber in combination with the vacuum within the drum causes the water to flow through the pulp mat. Additionally, resilient wipers may be attached to the slot to guide the water to the mat and keep the flow of water perpendicular to the mat, thus helping to force the water through the mat.
In operation, it has been discovered that the holes in the vertical portion of the plate in the upper chamber cause the water to impinge against the chamber sides and generate foam within the chamber. The foam represents the introduction of air into the water which when forced into the mat prevents the chemicals from being displaced from the mat and hampers the efficiency of the washer. The upper chamber also requires an inlet pipe to be connected to the top of the chamber, thus causing clearance problems with existing hoods which cover the washer and drum to contain any spray and protect workers. The lack of sufficient clearance requires the hoods to be cut away and reconstructed to accommodate such an inlet pipe.
To the extent that water is introduced into the chamber under pressure (to increase the efficiency of the washer) the top of the chamber is bolted on over a gasket and the top of the chamber is rounded, rather than flat, to help withstand pressure in the upper chamber. Leakage problems through the gasket have resulted from this usage of the upper chamber as a pressurized chamber. The need for a large number of bolts to secure the top cover of the chamber makes cleaning of the upper chamber difficult since each bolt must be removed to do so. In addition, the rounded structure of the top cover aggravates the hood clearance problem.
The resilient wipers used with the Uniflow washer have been attached to the slot at the bottom of the chamber with a number of nuts and bolts which run through holes drilled in the wipers. Replacing the wipers requires removal of all the nuts and bolts, which are typically placed approximately every three inches. The wipers have to be machined to add holes in the appropriate positions, which limits the materials which can be used for the wipers to those which can withstand and hold up after having holes drilled in them and additionally increases the cost of producing the wipers. A large number of bolts are required because the wiper pulls against each bolt, and the pulling force must be spread out to reduce tearing of the wiper due to such pulling forces.
The Uniflow washer is supported only at its ends. Depending upon the materials used, length of the washer, water flow, and so forth, it has been observed that the washer may tend to sag in the middle, thus producing an uneven distribution of water over the mat. Additionally, such sagging aggravates the clearance problem both above the mat and below the hood.
Another drawback of Uniflow washers arises when several washers are used for a single drum, with only one washer directly vertically above the drum. The other washers are positioned along the drum upstream of such vertical washer at an angle to the vertical. The steepness of such angle is limited because the water in the chamber will spill out at some point as the angle increases. This limitation on the distance between washers limits the time available for water to flow through the mat between washers, and thus limits the efficiency of the washers.